1. Field of the Invention
The invention relates to a novel negative electrode material and a method of manufacturing the same. More particularly, the invention relates to a high-capacity silicate negative electrode material and a method of manufacturing the same.
2. Description of the Related Art
In recent years, lithium secondary batteries have become increasingly commonplace as batteries having a high voltage and a high energy density. Owing to the expanded use of lithium secondary batteries in a broad range of fields and to demands for high performance, a variety of research is being carried out to further increase the performance of such batteries. For example, various materials are being investigated for use in negative electrodes, with carbon materials, aluminum alloys and the like seeing commercial application as negative electrode materials in practical batteries. However, although carbon materials are widely used because they are able to impart a high capacity, such materials have a low specific gravity and thus occupy a large proportion of the volume at the interior of the battery. Also, conventionally the performance of carbon materials has already been improved to a level where further improvements would be difficult to achieve. For this reason, achieving a higher capacity with negative electrode materials other than carbon materials is indispensable for increasing battery performance. At the same time, there exists a desire for the materials which make up a battery to be materials which are abundantly available on Earth.
Investigations on negative electrode materials other than carbon materials are being carried out to address this demand. For example, Japanese Patent Application Publication No. H10-12269 (JP-A 10-12269) discloses a solid electrolyte that has been imparted with, lithium ion conductivity, which electrolyte is composed of montmorillonite as the primary ingredient and contains therein a water-soluble lithium salt. Japanese Patent Application Publication No. H10-134814 (JP-A 10-134814) describes a lithium secondary battery which uses a carbon material obtained by inserting a carbon material precursor between the layers of a clay mineral having a layered crystal structure, such as montmorillonite, firing at from about 1,500° C. to about 2,000° C. in a vacuum or a nonreactive gas atmosphere so as to form a carbon material between the layers of the clay mineral, then dissolving and removing the clay mineral with an inorganic acid such as hydrofluoric acid.
In addition, Japanese Patent Application Publication No. 2004-296370 (JP-A 2004-296370) teaches a method of producing a negative electrode material for lithium secondary batteries, which method involves preparing a slurry of a layered mineral clay such as montmorillonite and a lithium salt so as to insert lithium ions between the layers of the layered clay mineral substance, then separating the layered clay mineral into which lithium ions have been inserted from the slurry to obtain a negative electrode material. As an illustrative example, this published patent application indicates that a lithium ion-containing montmorillonite which is crushed and heat-treated at 300° C. has a good discharge capacity. The powder x-ray diffraction pattern for this material is shown together with the powder x-ray diffraction pattern for the montmorillonite prior to heat treatment. However, even when the negative electrode materials shown as specific examples in JP-A 2004-296370 contained lithium ion-containing montmorillonite, the charge capacity was at most about 350 mAh/g.